1. Field of the Invention
The present invention concerns a process and a device to measure the signal quality of a digital information transmission system using a scanning device with adjustable direct current threshold to determine a signal parameter for the signal quality (Q-factor).
2. Description of the Related Art
To estimate the bit-error rate (BER) of a digital information transmission system the so-called Q-factor measuring method is applied, which further produces evidence concerning the signal-to-noise ratio (S/N) of a signal, the so-called eye pattern and the system reserve. Detailed clarifications about the measurement of the Q-factor can be found in a publication by the Hewlett-Packard company, xe2x80x98Measurement in High-Speed Optically Amplified Networksxe2x80x99 as part of the seminar papers of the xe2x80x981996 Lightwave Transmission Seminarxe2x80x99, which took place in May 1996 in Boeblingen at the Hewlett-Packard company and for example in the pamphlet of the ITU (International Telecommunication Union), Study-Period 1997 to 2000, COM 15-31-E, November 1997, Question 16/15, Study Group 15xe2x80x94 Contribution 31.
Up until now a bit-error rate of BER less than =10xe2x88x929 was considered to be sufficient for electrical digital information transmission systems. However, for optical digital information transmission systems a bit-error rate BER of  less than =10xe2x88x9212 is considered necessary. To carry out a measurement also for optical digital transmission systems using the methods usually applied when measuring an electrical digital information transmission system, the measuring time is too long.
In view of the fact that in future there will be so-called transparent optical networks whose data signals are not known and therefore the network operators will have to guarantee the quality of the system, there is a need for a process and a device to determine the transmission quality. Up until now known instruments to determine the transmission quality of digital information transmission systems require a known bit-pattern or additionally a second device to generate a clock signal. While the previously mentioned device is also designed with a purely numeric output of the bit-error rate the second device performs an oscillographic evaluation of an also unknown bit-pattern.
Generally circuits are known, which synchronize with an unknown input signal. An example for this is U.S. Pat. No. 5,757,857, which shows a circuit where the frequency difference between input signal and clock signal is synchronized with the data signal.
The present invention is based on the objective of suggesting a possibility to determine and report the quality of a digital information transmission system in case of unknown signals within a few seconds.
This task will be achieved according to the invention through a process as well as through a device as described hereinbelow. Further advantageous designs are also described hereinbelow.
According to the process the bit-error rate from an unknown input signal is determined and with its help a clock signal is generated with which the input signal is scanned using the scanning device. The unknown input signal is an electrical input signal, that is generated either out of an electrical digital information transmission system or through conversion of an optical digital information transmission system. From this electrical input signal the frequency of the bits per time is determined and the clock time signal is generated. The scanning device by which the input signal is scanned is based on the method of determining the bit-error rate (BER) by means of two deciders in terms of the sensing threshold as discussed initially.
According to a preferred realization of the process the clock generator is synchronized with the data frequency by means of a frequency sensitive synchronization circuit. This can for example be achieved by means of a search oscillator or by determining the bit-rate by the number per time of the positive or negative edges of the input signal. The latter is based on the notion that the edges of a quarter of all connected bit-blocks are positive in relation to the clock rate (refer to FREQUENZ 1970/8, page 230-234xe2x80x94in particular table 1xe2x80x94, xe2x80x98Eigenschaften und Anwendung von binaeren Quasi-Zufallsfolgenxe2x80x99 by Lutz Schweizer). Based on this realization the bit-rate can be inferred from the frequency of positive or negative edges multiplied by 4 and a corresponding clock signal can be generated.
For the scanning of the input signal with the scanning device a scanning oscillator is pre-adjusted to achieve exact synchronization following a further advantageous characteristic of the process by way of the bit-rate, which corresponds to the clock signal and then the scanning oscillator is synchronized through phase comparison to the input signal. This assists the fine adjustment, whereby it is adjusted in such a way that the center of the image i.e. the center of the digital impulses can be scanned.
With the device designed according to the invention an edge counter determines the bit-rate of the unknown input signal. A clock generator is linked to the edge counter, which receives a pre-adjustment signal from it, and which synchronizes through a phase locked loop with the scanning device by phase comparison to the input signal.
A device as well as a process designed according to the invention allows therefore the monitoring of electrical and optical digital information transmission systems while the bit-rate is unknown. Thereby, the bit-rate is reported numerically. In addition, a statement about the size of the Q-factor and all variables derived from it is possible. For this reason a measuring method was suggested (Q-factor) to measure the bit-error rate alternatively to shorten the measuring time considerably. Hereby, the known input signal is scanned simultaneously with two scanners, the first (reference scanner) scanning at optimum sensing threshold and regaining the clock time from the input signal, and the second (reading scanner) scanning with variable sensing threshold. The difference between both scanning outputs (=bit-errors) is detected with an EXOR depending on the sensing threshold of the reading scanner. This principle does not require the knowledge of the signal and can be used at the active information transmission system.
The process as well as the device is based on the fact that, as is especially common with optical transmission systems, a so-called NRZ-signal is being used as a transmission signal, where the signal level xe2x80x98HIGHxe2x80x99 corresponds to a digital xe2x80x981xe2x80x99 and the signal level xe2x80x98LOWxe2x80x99 corresponds to a digital xe2x80x980xe2x80x99.